The invention relates to a method of shutting down pressurized water reactors (PWRs) and more particularly to a method of shutting down PWRs at the beginning of scheduled refueling and/or maintenance outages at the end of their fuel cycles.
Commercial PWRs are employed to generate steam for driving electrical generators. FIG. 1 illustrates a commercial PWR 10 generally represented by a reactor pressure vessel (RPV) 12 having a core region 14 wherein heat is generated by fuel assemblies (not shown) containing fissile materials. The RPV 12 is the principal component of a reactor coolant system (RCS) 16, which may include from two to four coolant loops operating in parallel (which loops are represented by one coolant loop 18). Each loop 18 includes a steam generator 20 and a main reactor coolant pump (RCP) 22 for circulating the reactor coolant between the RPV 12 and the steam generator 20. The reactor coolant in PWRs is essentially highly pure water with closely controlled amounts of boron and lithium, which vary throughout the course of a fuel cycle. Thus, the reactor coolant may contain up to about 2500 ppm boron and up to about 3 ppm lithium. In addition, the reactor coolant may also contain 5-50 ppb zinc. A pressurizer vessel (PV) 24 is piped with one of the loops 18 for controlling the pressure of the RCS 16. The RCS 16 may operated at temperatures of up to about 600° F. or more and at pressures of up to about 2200 psi or more. The RCP 22 may have a 6,000-9,000 horsepower motor for circulating the reactor coolant at a rate of about 100,000 gpm against a head of up to 250 feet of water or more between the RPV 12 (where the heat of the nuclear reaction is absorbed by the reactor coolant) and the primary side, or tube side, of a steam generator 20 (where the heat is transferred from the reactor coolant to the secondary side, or shell side, and the steam is generated).
The materials of construction of RCS 16 parts wetted by the high temperature, high pressure, reactor coolant are selected based upon their superior mechanical properties and corrosion resistance. Thus, the vessel liners and most of the piping in the RCS 16 (including hot leg piping 30 and cold leg piping 32) are fabricated of stainless steel. The tubing 26 in the steam generators 20 and nozzle penetrations (not shown) in the removable heads 34 of the RPVs 12 are generally fabricated of Alloy 600, Alloy 690 or Alloy 800. All of these alloys generally contain nickel, chromium and iron. The structural members of the fuel assemblies (not shown) in the core region 14 are generally fabricated from zirconium-base alloys containing niobium, iron and tin.
Despite the fact that the RCS 16 materials of construction are highly resistant to general corrosion, thin oxide coatings (or films) develop over time on the thousands of square feet of RCS surface area wetted by the reactor coolant during power operations. Thus, oxide coatings develop outside of the core region 14 (or, simply, out-of-core) on the piping 30,32 and vessels 12,20,24. Portions of the oxide coatings then dissolve into the circulating reactor coolant or are released into the reactor coolant in particulate form (at which point the dissolved and particulate coatings are considered as undesirable corrosion products).
The dissolved and particulate corrosion products are transported by the reactor coolant throughout the RCS 16 during the fuel cycles. A portion of these transported corrosion products deposit on the out-of-core surfaces and another portion of these transported corrosion products deposit on the fuel assemblies in the core region 14 in the RPV 12 where they are activated by the neutron flux.
In recent years the nuclear industry has increased the boiling duties on the fuel assemblies. The increased amount of boiling heat transfer has led to increased deposition of corrosion products on the heat transfer surfaces of the fuel assemblies. These corrosion products may become undesirably thick and impede heat transfer from the tube to the bulk of the reactor coolant, which is expected to result in elevated tube cladding temperatures. The elevated temperatures may induce cladding corrosion or failure of the cladding material. For example, these increases may lead to an increased corrosion rate because of an increased concentration of reactor coolant solutes (principally lithium and boron) by boiling within the corrosion deposits on the fuel assemblies. This increased deposition of corrosion products on the heat transfer surfaces of the fuel assemblies also may induce power shifts in the core region 14 by concentration of boron within the deposits (a condition known as axial offset anomaly).
In addition, the activated corrosion products in the core regions 14 dissolve in circulating reactor coolant or are released into the reactor coolant in particulate form and are transported out of the RPVs 12. These activated corrosion products then redeposit on the wetted surfaces of the balance of the RCS 16 out of the RPV 12. Undesirably, these redeposited activated corrosion products cause a build up of radiation fields outside of the RPV 12 where technicians will be working in the course of the outages.
Thus, the nuclear industry desires to reduce the amount of corrosion products circulating in the RCSs in order to: operate the PWR without power shifts during power operations; reduce failures of the fuel assembly tubes; and reduce the radiation exposure of workers during outages.
The nuclear industry's primary method for removing corrosion products is to purify a slip stream (or side stream) reactor coolant purification system while the PWR is generating power or the PWR is shutdown. System 40 generally illustrates a system commonly known in the industry as the chemical and volume control system (CVCS), which system is designed to control the chemistry and radiochemistry of the reactor coolant. CVCSs 40 are designed to continuously circulate and purify a slip stream at a nominal rate of about 100 gpm. Thus, about 100 gpm of reactor coolant flows out of the RCS 16 through piping 42, through a heat exchanger 44 for heating purified reactor coolant returning to the RCS 16 through return piping 46, through a water cooled heat exchanger 48, through an ion exchange vessel 50 for trapping corrosion products (including activated corrosion products) and a filter 51, and into a volume control tank (VCT) 52. In state of the art PWRs that maintain residual hydrogen concentrations in the reactor coolant during power operations, the VCTs 52 have a hydrogen gas blanket over the reactor coolant for maintaining the dissolved hydrogen concentration within a desired range.
The reactor coolant in the VCT 52 is then pumped by a positive displacement charging pump 54 at a nominal rate of about 100 gpm through the heat exchanger 44 and the piping 46 back to the RCS 16. In addition, boron in the form of boric acid may be made up as an aqueous solution in a boric acid feed tank 56 and pumped by a centrifugal feed pump 58 into the CVCS 40 for varying the boron concentration and thereby controlling the nuclear reaction in the RCS 16. Similarly, zinc in the form of zinc acetate or zinc borate may be made up as an aqueous solution and pumped into the CVCS 40 for developing and later maintaining a tight oxide coating on the wetted RCS surfaces and thereby reducing radiation levels and inhibiting stress corrosion cracking in the RCS 16. See e.g., U.S. Pat. Nos. 5,108,697 and 5,171,515.
At the end of the fuel cycles, substantial amounts of corrosion products remaining in RCSs 16 are removed in the course of refueling outages when spent fuel assemblies (and the corrosion products deposited thereon) are removed and replaced with fresh fuel assemblies. In addition, shutdown processes (including a state of the art shutdown process to be discussed in detail below) have been employed in attempts to remove additional amounts of corrosion products via the CVCSs 40 in a reasonable amount of time before removing the spent fuel assemblies. However, known commercial shutdown processes can not substantially reduce radiation fields generated in out-of-core oxidized coatings.
It has been estimated that less than about ten percent of the total amount of the corrosion products in a PWR are removed by the CVCSs 40, by commercial shutdown processes and by removal of the spent fuel assemblies. Thus, these steps may not be sufficient to prevent the power shifts experienced at higher fuel duties, prevent fuel failures or facilitate shutdowns with reduced radiation exposures.
Various decontamination methods for removing corrosion products (including activated corrosion products) from RCSs 16 have been proposed which would be applied to the RCSs 16 after the PWRs has been shutdown. See generally, Electric Power Research Institute Report NP-1168, entitled “Plant Decontamination Methods Review”, May 1981; and M. E. Pick et al., “Chemical Decontamination Of Water Reactors, CEGB Developments And The International Scene”, Nuclear Energy, Vol. 22, No.6, December 1983, pp.433-444. “Dilute chemical” decontamination methods (i.e., methods involving the use of aqueous solutions containing about one percent or less decontamination reagents) have been considered by the nuclear industry for decontaminating RCSs 16 with fuel assemblies permitted to remain in the core regions 14 of the reactor pressure vessels (RPVs). Advantageously, these methods can be implemented using the reactor coolant in the course of the decontamination steps and by connecting temporary decontamination systems to spool pieces 68 connected with the PWRs' residual heat removal systems (RHRSs) 70. RHRSs 70 generally include centrifugal circulation pumps 72 and heat exchangers 74 for (among other things) cooling the circulating reactor coolant while the PWRs 10 are shutdown and the steam generators 20 are isolated from the balance of the RCSs 16.
Two dilute chemical decontamination methods have been qualified and successfully used to decontaminate commercial PWRs or selected subsystems and components, including the LOMI (Low Oxidation State Metal Ion) Process licensed by the Electric Power Research Institute and the Can-Derem (and its predecessor Can-Decon) Process licensed by the Atomic Energy of Canada, Ltd. Both of these processes utilize a decontamination reagent including a reducing agent and a chelant. A “chelant” is a coordination compound having a central atom joined to two or more other atoms of one or more other molecules or ions (sometimes called ligands) such that heterocyclic rings may be formed with the central atom as part of each ring. Organic acids and their salts may be employed as chelants. LOMI reagents generally comprise vanadous picolinate in aqueous solution. One LOMI reagent is a mixture generally comprising about 0.006 molar vanadous formate, 0.036 molar picolinic acid and sufficient sodium hydroxide to adjust the pH to a value between 4 and 5. Can-Derem reagents generally comprise ethylene diamine tetra acetate (EDTA) and citric acid in aqueous solution. One Can-Derem reagent is a mixture generally comprising 0.1 wt. % total sum of EDTA and citric acid and having a pH of between 2.3 and 3. These reducing agents are generally effective in reducing iron and nickel ions in the RCS oxide coatings from the +3 valence state to the +2 valence state in reasonable amounts of time. The LOMI reagents are normally more effective than the Can-Derem reagents in reducing the nickel ions. See, e.g., U.S. Pat. Nos. 4,470,951; 5,089,216 and 5,805,654.
Neither the LOMI Process nor the Can-Derem Process can effectively reduce chromium in RCS oxide corrosion products in a reasonable time period. Accordingly, the LOMI and Can-Derem Processes are normally performed as a series of steps with alternating chromium oxidation steps for oxidizing the chromium in the oxide coatings from the +3 valence state to the +6 valence state. One chromium oxidation process that has been qualified by the nuclear industry is the “AP” (or Alkaline Permanganate) Process, which utilizes a basic aqueous solution of potassium permanganate as an oxidizing agent with sodium hydroxide for controlling the pH. In a subsequent substep, oxalic acid or other reducing agent may be added to decompose the residual peroxide to protect the ion exchange resins. See, e.g., U.S. Pat. No. 5,278,743. Other chromium oxidation processes proposed by the nuclear industry include the NP and POD Processes, both of which employ aqueous solutions of potassium permanganate with nitric acid instead of a basic solution. These combined processes can remove up to 85% or more of the corrosion products on the wetted surfaces of RCSs 16.
Table 1 compares the key results of a full system decontamination of a commercial PWR having four loops 18 that was performed with the fuel assemblies removed from the core region 14 (including a five-step decontamination process (Can-Derem_AP_Can-Derem_AP_Can-Derem)) with the results of a state of the art shutdown process that was performed by the assignee of the present invention at several commercial PWRs having four loops 18:
TABLE 1ResinCuriesVolume (ft3)TimeProcessRemovedGenerated(hrs)Fuel-out 5-step10,0002,600104DecontaminationProcessState Of The Art2,000-3040-Shutdown Process6,00060Table 1 shows that LOMI and Can-Derem chemical decontamination processes can be expected to remove substantially more activated corrosion products (and non-activated corrosion products) than can known shutdown processes (exemplified by the assignee's state-of-the-art process), but will generate considerably more waste and require considerably more time. Other PWR 10 decontamination applications have involved subsystems such as the RHRSs 70 or components such as steam generator channel heads 78. See, e.g., U.S. Pat. No. 5,517,539, which discloses a method of decontaminating a steam generator channel head 78.
The state of the art shutdown process was designed by the assignee of the present invention as part of an effort to complete refueling outages in less than about 20 days and to limit the radiation exposures of workers to less than 100 person rem, which is lower than exposures obtained with previously employed shutdown processes. It is noted that the public literature indicates that the shortest quartile of refueling outages in the United States in 2002 were completed in 23 days or less. Accordingly, the nuclear industry now expects future shutdowns to remove substantial quantities of corrosion products (including activated corrosion products) in approximately the same period of time as the state of the art shutdown process with accumulated radiation exposures of less than 100 person rem. In addition, the industry desires to decontaminate its PWRs with less wastes than were generated by the earlier full system decontamination and at much lower cost.